1. Field of the Invention
The present invention relates to a sheet-thickness detector device for detecting the thickness of a conveyed sheet and a sheet-processing apparatus, an image-forming apparatus having the sheet-thickness detector device.
2. Description of the Related Art
An image-forming apparatus for forming images on a sheet, such as a copying machine, a facsimile machine, a printer, and a multifunction machine combining these functions may have been provided with a sheet-processing apparatus (finisher) in an apparatus body as one of its components.
In the sheet-processing apparatus, after sheets discharged from a recording unit for recording images on a sheet of an image-forming apparatus are received and stacked in a bundle, at least one of processes of lateral arrangement by jogging the bundle, punching to perforate the sheets, stapling to bind the sheets, and folding to fold the sheets is performed on the sheets.
Recently, the sheet-processing apparatus frequently includes a sheet-inserting device called as an inserter or interposer. The sheet-inserting device functions to insert an insert-sheet prepared in advance, such as a cover and an insert sheet, onto a predetermined page of the recorded sheet bundle, such as the top page, the last page, or a middle page.
In particular, one such interposer which is becoming widely used is one, in which insert-sheets prepared in advance are inserted onto one place or a plurality of places of a sheet bundle. The interposer provides for implementation of a monochrome/color mixed document at high speed and low cost when the interposer is attached to a recording unit for recording monochrome images on a sheet so as to insert color pages at mid-points of the sheet bundle.
In such an interposer, insert sheets stacked in a bundle in the inserting order by an operator are sequentially supplied at the predetermined timing and allowed to be inserted between sheets discharged from the recording unit, so that a desired sheet bundle can be prepared by the sheet-processing apparatus.
In the interposer, however, during the supplying of insert-sheets from a bundle prepared in advance, if two or more sheets are supplied at a time, i.e., so-called double feeding, a problem arises that the insert-sheets get out of order thereafter, so that after the double feeding, the sheet bundle is not a usable sheet bundle. Moreover, if the stable processing or the punching in the sheet-processing apparatus is performed on the sheet bundle, the sheet bundle cannot be reused so that it must be discarded.
Then, a conventional interposer has been provided with a sheet-thickness detector device for preventing the double feeding of the insert sheet. The sheet-thickness detector device measures the thickness of an insert sheet with a magnetic element and a magnetic field sensor and analyzes the result to determine if it is double fed. The sheet-thickness detector device of this system is widely used because it is small in size and low in cost. An example of this system is disclosed in Japanese Patent Laid-Open No. 2000-146510.
FIG. 11 shows a skeletal structure of the sheet-thickness detector device disclosed in the above publication. In a sheet-thickness detector device 11, an insert sheet P conveyed on a guide plate 13 by a feed roller 12 is pulled through between a magnetic field sensor 14 and a magnetic element 16 urged by a spring 15 toward the magnetic field sensor 14, so that the sheet thickness is detected by detecting changes in magnetism with the magnetic field sensor 14.
Incidentally, in the conventional sheet-thickness detector device 11, the magnetic field sensor 14 is provided on the bottom of the guide plate 13, so that the guide plate 13 exists between the magnetic field sensor 14 and the magnetic element 16. Therefore, in the conventional sheet-thickness detector device 11, the space between the magnetic field sensor 14 and the magnetic element 16 is increased by the thickness of the guide plate 13, correspondingly reducing accuracies in sheet-thickness detection. Also, as the guide plate 13 of the conventional sheet-thickness detector device 11, a material with high permeability has to be used; the material of the guide plate 13 is limited.